Late night radio history lesson: Project ELF
Several countries have explored or used Extra Low Frequency (ELF) communication systems, primarily for military purposes, particularly for submarine communications.
These ELF systems are primarily used for military purposes due to their ability to reach deep-sea submarines. The use of ELF by these countries reflects the strategic importance of maintaining secure and reliable communication with submarine fleets, especially those carrying ballistic missiles as a part of nuclear deterrence strategies.
In addition to making this piece on Project ELF, KB9VBR is a great source of information on ham radio, and operating portable.